They're not easily accessible. If you run the 'meteor bundle' command, you can generate a tar.gz file, and then run your app manually. Doing that, you should be able to access the mongo logs... probably in the .meteor/db directory. If you really need to access mongodb log files, set up a regular mongodb instance, and then connect Meteor to an external mongo instance, by setting the MONGO_URL environment variable:
Once that's done, you should be able to access logs in the usual places...